The temperature of a catalytic converter is one of the most significant factors which affects its efficiency. The efficiency drops off rapidly at both high and low temperatures, leaving a relatively narrow band of operating temperatures within which efficiency is highest.
One known method of controlling the temperature of a catalytic converter is to provide a heater and one version of such a system is disclosed in EP 0570241.
For hybrid vehicles, the use of a heated catalytic converter is further developed by providing power to the heater from regenerative braking and one such system is known from EP 0511654.
Apart from providing a special heater, one other way of controlling the temperature of a catalytic converter is by controlling the temperature of the exhaust gases passing through it. This is difficult to achieve because the exhaust gas temperature is primarily a function of the load on the engine, which cannot normally be controlled. Factors affecting catalyst performance are particularly important in compression ignition engines, such as diesel engines, where it is difficult to modify fuelling so as to attain optimum conversion efficiency, as is done in spark ignition engines, such as petrol engines.
A further problem with controlling catalytic converter temperatures is that the efficiency of modern engines is increasing, with the result that they produce less waste heat in the exhaust system. This slows down the rate at which catalytic converters can be heated up to a suitable operating temperature and can also mean that the operating temperature is not maintained while the load on the engine is low, for example when it is at idle.
In U.S. Pat. No. 5,545,928 a method of controlling catalytic converter temperature is disclosed for a series hybrid vehicle, which is a type of vehicle comprising an engine which drives an electrical machine and the electrical machine provides all the torque to the driven wheels. The system controls the rate of generation of electrical power to achieve various results, including increasing the temperature of the catalytic converter. In series hybrid vehicles the engine is usually tuned to deliver optimum performance under tightly controlled conditions and it is a problem of this type of arrangement that it is difficult to finely control the rate at which electrical power is generated so as to reduce the load on the engine in order in turn to reduce the temperature of the catalytic converter, without also compromising overall efficiency.